1. Field of the Invention
The present invention relates to electrographic printing. More particularly, the invention relates to efficient removal of volatile organic compounds from the exhaust of an electrographic printer.
2. Background of the Art
Electrophotography forms the technical basis for various well-known imaging processes, including photocopying and some forms of laser printing. Other imaging processes use electrostatic or ionographic printing. Electrostatic printing is printing where a dielectric receptor or substrate is “written” upon imagewise by a charged stylus, leaving a latent electrostatic image on the surface of the dielectric receptor. This dielectric receptor is not photosensitive and is generally not re-useable. Once the image pattern has been “written” onto the dielectric receptor in the form of an electrostatic charge pattern of positive or negative polarity, oppositely charged toner particles are applied to the dielectric receptor to develop the latent image. An exemplary electrostatic imaging process is described in U.S. Pat. No. 5,176,974.
In contrast, electrophotographic imaging processes typically involve the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, cleaning, and erasure.
In the charging step, a photoreceptor is covered with a charge of a desired polarity, either negative or positive, typically with a corona charge applicator or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively exposing the photoreceptor to electromagnetic radiation, thereby discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. The electromagnetic radiation, which may also be referred to as “light”, may include infrared radiation, visible light, and ultraviolet radiation, for example.
In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using a developer electrically-biased to a potential opposite in polarity to the toner polarity. The toner particles migrate to the photoreceptor and selectively adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to affect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. The transfer of an image typically occurs by one of the following two methods: elastomeric assist (also referred to herein as “adhesive transfer”) or electrostatic assist (also referred to herein as “electrostatic transfer”).
Elastomeric assist or adhesive transfer refers generally to a process in which the transfer of an image is primarily caused by balancing the relative energies between the ink, a photoreceptor surface and a temporary carrier surface or medium for the toner. The effectiveness of such elastomeric assist or adhesive transfer is controlled by several variables including surface energy, temperature, pressure, and toner rheology. An exemplary elastomeric assist/adhesive image transfer process is described in U.S. Pat. No. 5,916,718.
Electrostatic assist or electrostatic transfer refers generally to a process in which transfer of an image is primarily affected by electrostatic charges or charge differential phenomena between the receptor surface and the temporary carrier surface or medium for the toner. Electrostatic transfer may be influenced by surface energy, temperature, and pressure, but the primary driving forces causing the toner image to be transferred to the final substrate are electrostatic forces. An exemplary electrostatic transfer process is described in U.S. Pat. No. 4,420,244.
In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed. Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Electrophotographic imaging processes may also be distinguished as being either multi-color or monochrome printing processes. Multi-color printing processes are commonly used for printing graphic art or photographic images, while monochrome printing is used primarily for printing text. Some multi-color electrophotographic printing processes use a multi-pass process to apply multiple colors as needed on the photoreceptor to create the composite image that will be transferred to the final image receptor, either via an intermediate transfer member or directly. One example of such a process is described in U.S. Pat. No. 5,432,591.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated to some degree in the liquid carrier (or carrier liquid); typically in more than 50 weight percent of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron, liquid toners are capable of producing very high-resolution toned images, and are therefore preferred for high resolution, multi-color printing applications.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like. Exemplary liquid toners and liquid electrophotographic imaging process are described by Schmidt, S. P. and Larson, J. R. in Handbook of Imaging Materials Diamond, A. S., Ed: Marcel Dekker: New York; Chapter 6, pp 227–252.
The liquid toner composition can vary greatly with the type of transfer used because liquid toner particles used in adhesive transfer imaging processes must be “film-formed” and have adhesive properties after development on the photoreceptor, while liquid toners used in electrostatic transfer imaging processes must remain as distinct charged particles after development on the photoreceptor.
Toner particles useful in adhesive transfer processes generally have effective glass transition temperatures below approximately 30° C. and volume mean particle diameter between 0.1–1 microns. In addition, for liquid toners used in adhesive transfer imaging processes, the carrier liquid generally has a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto a photoreceptor, transfer belt, and/or receptor sheet. This is particularly true for cases in which multiple colors are sequentially deposited and overlaid to form a single image, because in adhesive transfer systems, the transfer is promoted by a drier toned image that has high cohesive strength (commonly referred to as being “film formed”). Generally, the toned imaged should be dried to higher than approximately 68–74 volume percent solids in order to be “film-formed” sufficiently to exhibit good adhesive transfer. U.S. Pat. No. 6,255,363 describes the formulation of liquid electrophotographic toners suitable for use in imaging processes using adhesive transfer.
In contrast, toner particles useful in electrostatic transfer processes generally have effective glass transition temperatures above approximately 40° C. and volume mean particle diameter between 3–10 microns. For liquid toners used in electrostatic transfer imaging processes, the toned image is preferably no more than approximately 30% w/w solids for good transfer. A rapidly evaporating carrier liquid is therefore not preferred for imaging processes using electrostatic transfer. U.S. Pat. No. 4,413,048 describes the formulation of one type of liquid electrophotographic toner suitable for use in imaging processes using electrostatic transfer.
Photoreceptors generally have a photoconductive layer that transports charge (either by an electron transfer or charge transfer mechanism) when the photoconductive layer is exposed to activating electromagnetic radiation or light. The photoconductive layer is generally affixed to an electroconductive support, such as a conductive drum or an insulative substrate that is vapor coated with aluminum or another conductor. The surface of the photoreceptor can be either negatively or positively charged so that when activating electromagnetic radiation strikes certain regions of the photoconductive layer, charge is conducted through the photoreceptor to neutralize, dissipate or reduce the surface potential in those activated regions.
Many electrophotographic imaging processes make use of intermediate transfer members (ITM's) to assist in transferring the developed toner image to the final image receptor. In particular, in a multipass electrophotographic process, these ITM's may contact the final image formed on the photoreceptor to assist transfer of entire image to transfer to the ITM. The image may then be transferred from the ITM to the final image receptor, typically through contact between the ITM and the final receptor.
In a tandem process, individual photoreceptors layer the images formed by the component colors on the ITM. When the entire image is composed in this manner it is typically transferred to the final image receptor. U.S. Pat. No. 5,432,591, for example, discloses the use of an offset roller to remove the entire image from a photoreceptor and transfer it to the final image receptor in a multi-pass liquid electrophotographic process. In various embodiments, the ITM may be an endless belt, a roller or a drum.
One continuing problem in liquid electrophotography is that the hydrocarbon liquid developers are highly volatile and fairly high concentrations of hydrocarbon molecules, called VOC's (volatile organic compounds), may enter the ambient air during the electrographic process and may constitute an environment hazard. Some airborne VOC's are produced by natural evaporation in and around the electrographic drum or belt, some from the dispensing of the liquid carrier, and some from evaporation of the liquid during fusing of the print to the sheet when the image is dried and this is most notably the biggest cause for the evaporation. This creates an environmental hazard during continued use that becomes more severe with increased usage and speeds of the printer. In addition, the evaporation of the VOC's also may cause a very objectionable smell. In liquid electrophotography, the hydrocarbon vapors must be captured not only at the fusing operation of the process but at all points where there are fugitive emissions of the hydrocarbon vapors.
The World Health Organization definition of VOCs includes all organic compounds (substances made up of predominantly carbon and hydrogen) with boiling temperatures in the range of 20–260° C., excluding pesticides. This means that they are likely to be present as a vapor or gas in normal ambient temperatures. People are exposed to the VOC's by breathing the contaminated air. The health effects depend on the specific composition of the VOC's present, the concentration, and the length of exposure. High concentrations of some compounds could have serious health effects. General effects include eye, nose and throat irritation, headaches, loss of coordination, nausea, damage to the liver, kidneys and central nervous system and some are suspected or known to cause cancer in humans.
The Occupational Safety and Health Association (OSHA) has formulated PELs (permissible exposure limit) and IDLHs (immediately dangerous to life of health) for separate organic compounds but not for total VOC exposure. The American Conference of Governmental Industrial Hygienists (ACGIH) has established TLV-TWA (Threshold Limit Value-Time Weighted Average) values and STEL (Threshold Limit Value, Short Term Exposure Limit) for single compounds but do not take into account the effects of simultaneous or serial exposure to complex mixtures and has not established values for total VOC exposure. The World Health Organization has not established guidelines for total VOC exposure.
The art continues to search for methods and apparatus for economically and efficiently reducing the VOC's from hydrocarbon carriers in liquid electrographic printers. For example, one approach involves condensation of the hydrocarbon vapors (refer to U.S. Pat. No. 5,737,674). Alternatively, the art teaches absorption or adsorption of the hydrocarbon vapors into another material such as carbon (refer to U.S. Pat. No. 4,760,423). Another alternative involves catalytic oxidation of the hydrocarbon vapors to carbon dioxide and water (refer to U.S. Pat. Application Publication US2004/0047645A1).
Background information on catalytic oxidation is documented in Hodnett, Kieran K. and Hodnett, B. K., Heterogeneous Catalytic Oxidation: Fundamental & Technological Aspects of the Selective & Total Oxidation of Organic Compounds, John Wiley & Sons Inc., 2000 and Warren, Barbara K and Oyama, Ted S., Heterogeneous Hydrocarbon Oxidation, Oxford University Press, 1996, and Spivy, James J., “Complete Catalytic Oxidation of Volatile Organics”, Industrial Engineering and Chemistry Research, 26, pp 2165–2180 1987, and Journal of Hazardous Materials, Vol 109, Issue 1–3, p 113, June 2004.